This invention relates to spark ignited internal combustion engines of the type including means to sense engine knock and control an engine variable such as spark timing in closed loop in response to the sensed engine knock to prevent the engine knock intensity from exceeding a predetermined level. Such engines may be equipped with turbochargers or provided with higher compression ratios or advanced spark timing for greater power and fuel economy with the assurance that engine operating conditions conducive to severe knock will not damage the engine or annoy the vehicle operator.
One of the difficulties involved in the sensing and control of knock in such engines is the signal-to-noise ratio of the sensed knock signal. Knock is generally sensed in such systems by means of an electromechanical vibration or detonation transducer which is physically attached to an engine component to respond to the knock induced ringing vibrations of the engine. However, since the modes and characteristic frequencies of these ringing vibrations are determined by the engine structure and there are many other vibration generating events that occur during the operation of such an engine, prior art methods of signal processing based on simple frequency or amplitude discrimination have rarely been successful in providing commercially acceptable knock control for a mass produced, vehicle mounted engine. Only one such system, described in U.S. Pat. No. 4,111,035, issued to Gene A. West and Glen C. Hamron on Sept. 5, 1978, has shown such commercial success; and even this system, on some engines and under some operating conditions, may occasionally allow a brief burst of excessive knock or respond to noise other than knock and produce a condition known as false retard, the name of which condition is derived from the retard of spark timing which is the means by which excessive knock is reduced when it is sensed.
A superior new system for controlling knock using a demodulator and a low pass filter is described in the copending patent application U.S. Ser. No. 149,645, filed by Herman Brandt on the same date as this application and assigned to the same assignee. This system uses a low pass filter which approximates the impulse response of a matched filter for the envelope of a typical knock burst and provides an improvement in signal-to-noise ratio by suppressing bursts or pulses not due to knock which are characterized by the characteristic knock carrier frequency of the engine and have amplitudes comparable to knock induced pulses but have a different envelope shape over time.
One consistent difference between knock induced and other vibration bursts in such engines is the time duration of the burst. It has been observed that most knock induced pulses, at least for strong or intense knock of the type that requires control, have time durations of approximately three to ten milliseconds, whereas most other engine vibration bursts appear to last for a significantly shorter time. Therefore, after the output of the low pass filter in the aforementioned superior knock control system has been compared with a reference in a comparator to produce well-formed, constant amplitude knock pulses, the signal-to-noise ratio might be improved even more in some cases by the act of ignoring or suppressing all such pulses having a duration less than the lowest normally expected for engine knock. Because of the time extending effect of the low pass filter on all pulses passing therethrough, output pulses from the comparator of less than some duration greater than one millisecond could be ignored. One method which we have developed to accomplish this is to delay the rising edge of each such comparator output pulse by two milliseconds. This will cause all pulses of less than two milliseconds duration to disappear, but it will also shorten all other pulses by the same two millisecond duration. This last effect tends to aggravate another problem of such knock control systems, which is the incompatibility between control of moderate and control of strong knock.
In a knock control system in which engine knock pulses are themselves measured and the duration of such pulses is taken as a measure of the severity of the knock, there is not necessarily a proportional correlation between the durations of said knock pulses and the amount of correction required for the engine control variable which reduces knock. In a system such as that described above, it has been found that the variation in knock pulse duration between moderate, less intense knock and severe, more intense knock is often greater than the variation in required corrections to the spark timing of the engine for reduction of the appropriate knock. More specifically, if the system is adjusted to control moderate knock, severe knock would provide too much spark retard, possibly leading to a noticeable and unnecessary loss of power or surge as the spark is retarded beyond the required amount. However, if the system is adjusted for control of severe knock, the amount of retard for moderate knock is insufficient and such knock may exceed the maximum desired audible level. The use of an ignore timer as described above aggravates this problem by increasing the ratio between time durations of moderate and severe knock bursts or pulses.